Verification of quantum communication protocol implementation security

Abstract

A verification assembly (100) for verifying the security of a quantum protocol, QKD, comprises: a quantum channel input (110); a routing quantum channel (130); a quantum channel output (120); a connection (140) for an eavesdropping line (140), which is intended to be connected to a public communication channel; and a communication connection (150) for a communication line (150), which is intended to be connected to an external device in bidirectional communication. A corresponding arrangement and a method for verifying a QKD protocol to be tested are also proposed.

Description

[0001] Federal Printing Office GmbH

[0002] BD-G0121-24 WO

[0003] Verification of the security of the implementation of quantum communication protocols

[0004] Field of invention

[0005] The present invention relates to the field of quantum communication, in particular key exchange, and especially the verification of the security of quantum communication protocols.

[0006] background

[0007] A protocol for conducting a quantum key exchange or a quantum communication protocol can be implemented, as shown in Fig. 7, by an arrangement of a transmitter 710, which is communicatively connected to a receiver 720 via a quantum channel 730 and a public channel 740. The transmitter 710 generates qubits in a random generator 715, which are randomly modified in a subsequently arranged random modifier 717. The transmitter 710 has a memory (not shown) in which the bit value of the randomly generated qubit is stored, along with the value of the transmitter-side modifier.

[0008] The modified qubits are transmitted via the quantum channel 730 to the receiver 720 and randomly modified by a randomly controlled modifier 1 of the qubits. The received and receiver-side randomly modified qubits are then measured by a measuring device 725. The measured value is passed on as a bit to a storage device (not shown) in the receiver 720 and stored together with the receiver-side modification.

[0009] It was previously described that the transmitter 710 randomly modifies randomly generated bits in a random modifier 717 in the random generator 715, and that the received qubit is randomly modified in a random modifier by the receiver 720 and then measured. Equivalently, this process can be described as follows: The transmitter 710 randomly selects a bit value x (x = 0 or 1) (generator 715) and subsequently (modifier 717) another random bit b. A (0 or 1), which determines one of two orthogonal bases used to encode the bit as a quantum state. The qubit is sent from sender 710 via quantum channel 730 to receiver 720, where the receiver randomly selects a base b. B chosen one of two mutually orthogonal bases to measure the received quantum state and store the binary result.

[0010] A physical quantum usually serves as the physical representation of the information technology unit of a qubit. Since a qubit can be realized by different quanta, for example a photon with a specific polarization or an electron with a specific spin, the terms quantum and qubit are used synonymously here, unless otherwise noted, to emphasize the universality of the representation.

[0011] 2025-11-14 BD-G0121-24 WO - Description: Via public channel 740, transmitter 710 and receiver 720 can compare the respective modifier settings stored on the transmitter side with the respective modifier settings stored on the receiver side, according to the transmitted sequence of the transmitted qubits.

[0012] Such arrangements are known, for example, from the description of the BB84 protocol. One possible implementation involves the qubits being realized by specifically polarized photons and the respective modifier being realized by a polarization filter with a randomly adjustable polarization angle – in the case of photonic qubits, this is, in other words, the basis.

[0013] The associated procedure 800 is illustrated in Fig. 8: The procedure begins in step 810. In step 820, a sequence of qubits is sent from the transmitter 710 to the receiver 720 via the quantum channel 730. Each transmitted qubit is generated from a randomly generated bit and converted into a qubit, 715, and is subjected to a random modifier setting in the modifier 717 before it enters the quantum channel 730. Once the qubit has arrived at the receiver 720, it is forwarded by the receiver-side modifier 727 with a random modifier setting to the measuring device 725, which forms a transmissible bit from the measured qubit, provided it has arrived.

[0014] In step 830, transmitter 710 and receiver 720 exchange information about their modifier settings via public channel 740. This means exchanging the sequence of generated and received events, which consists of a sequence of bit values ​​and modifier settings on both sides. Only if the modifier settings are identical—and provided a qubit could be measured at the measuring device 725—is the corresponding bit included in the raw key on both sides. In some embodiments, the modifier settings can be identified by their respective transmitter and receiver bases. If the transmitted qubits are represented by polarized photons, the respective base can be one of several positions of an optical polarization filter.

[0015] This raw key may be faulty, for example, due to noise or fluctuations in the settings of the respective transmitter and / or receiver device. The raw key may also be faulty if an eavesdropping party, such as a potentially present eavesdropper E, 731, has attempted to read the qubit, because reading it causes the multitude of possibilities that the qubit could potentially assume to collapse into a reliably measured value. This potential interference by the eavesdropper or eavesdropping party E, 731 is symbolized by the dashed section 732 of quantum channel 730. The intercepted qubit in question is therefore unsuitable for transmission to the receiver or would have to be regenerated in a forged manner. This situation is detected by both sender and receiver due to the comparison between them, and the presence of an unauthorized eavesdropping party thus becomes obvious.

[0016] In step 840, an error estimate is therefore performed. This can be done, for example, by sacrificing a number of bits. Then, for instance, a small portion of a relatively large number of transmitted bits (the qubits exist as classical bits before transmission and after reception processing) is publicly compared bit by bit. This portion of the raw key is then no longer suitable for generating the actual key. Ideally, with a perfect quantum link, ideal transmitting and receiving equipment, and the absence of an eavesdropping party, all bits would be identical. In practice, deviations will occur, leading to a measured error rate, which is then used as an estimate for all transmitted bits.

[0017] 2025-11-14 BD-G0121-24 WO - Description: Quantum Bit Error Rate (QBER). Naturally, the lowest possible QBER is desirable.

[0018] For each QKD protocol, a critical quantum error rate, known as a critical QBER (cQBER), can be specified. For example, for the BB84 protocol, this is 11%, i.e., cQBER | B B84=11%. In step 850, the corresponding comparison is made to determine whether the estimated QBER is smaller than the minimum permissible critical value cQBER for the QKD under consideration. If the estimated QBER is less than cQBER, the remaining bits of the raw key could be used as a secure key, in step 860. However, if the QBER determined in step 840 is, for example, higher than the value of cQBER | BB84, in which case the key would be completely discarded and measures would have to be taken in step 870. These measures could include, but are not limited to: a new attempt at a new key agreement, an examination of the equipment components for inaccuracies in their settings, an examination of the environment for interference, and an examination of the quantum link for possible eavesdroppers. In any case, the procedure ends in step 880.

[0019] The publication “QKD as a Quantum Machine Learning task” by Decker, T. et al., arXiv:2410.01904vl 2 Oct 2024 describes how QKD protocols are proposed as a use case for QML algorithms.

[0020] In contrast, this paper proposes a verification of the security of the implementation of quantum communication protocols.

[0021] Summary of the invention

[0022] A verification module for verifying the security of a quantum protocol (QKD) is proposed. The term "quantum protocol" refers specifically to a quantum communication protocol and can be equivalently denoted as QKD, or Quantum Key Distribution. The verification module can be used, in particular, to support the verification of quantum protocols in a laboratory setting. It is intended that general quantum protocols can be verified. With regard to known protocols such as BB84, the verification module can be used to verify the instrumental implementation of the otherwise well-understood BB84. With regard to quantum communication protocols that have not yet been fully investigated, the verification module can be used to test their quality.

[0023] The terms quantum computer and computer are described in detail elsewhere in this application. However, it should also be understood that the verification assembly does not necessarily have to be embodied; rather, providing a computer program product that simulates the functionalities of the verification assembly is also being considered.

[0024] The verification assembly comprises: a quantum channel input; a forwarding quantum channel; a quantum channel output; a connection for a listening line, intended to be connected to a public communications channel; and a

[0025] 2025-11-14 BD-G0121-24 WO - Description: Communication port for a communication line intended to be connected to an external device in bidirectional communication.

[0026] The verification assembly can be configured to receive a qubit via the quantum channel input. The forwarding quantum channel can be configured to forward the received qubit. Furthermore, the quantum channel output can be configured to retransmit the received qubit. To verify a quantum communication protocol, in addition to the quantum channel, a public channel is required, through which two parties would typically exchange information, for example, about key quality or key correction details. The verification assembly also has a communication interface through which it can send information to a monitoring instance or receive commands from it. Since the verification device is preferably used in a laboratory, the monitoring instance can be the laboratory itself.

[0027] In embodiments, the verification assembly may further comprise: an internal qubit generator designed to generate a probe qubit within the verification device; a quantum computer designed to entangle a QKD qubit received via the quantum channel input with the probe qubit; an internal qubit sensor designed to measure the entangled resulting output probe qubit internally.

[0028] The term "internal" here refers specifically to conditions or components within the verification assembly. The internal qubit generator serves to create a qubit internally for further processing. It is also conceivable that the internal qubit generator is capable of creating more than one qubit simultaneously. The qubit generated by the internal qubit generator is referred to below as the probe qubit. Furthermore, the verification assembly may include a quantum computer. The quantum computer can also be viewed as an array of quantum circuits or an array of quantum gates. The quantum computer serves to entangle the qubit received via the quantum channel input, which can also be called the QKD qubit, with the probe qubit. The result of the entanglement is the output probe qubit. The verification assembly also includes an internal qubit sensor.This serves to measure the output probe qubit. Based on this measurement, the verification assembly can deduce the QKD qubit received via the quantum channel input. How the entanglement can be achieved in detail is discussed elsewhere in this application.

[0029] In some embodiments, the internal qubit generator may be configured to produce random probe qubits. This can be advantageous because it allows for the simulation of a quantum protocol to be verified.

[0030] In embodiments, the verification assembly may further include: An internal correlation device configured to derive an estimate for a QKD key from the measured output probe qubit and the captured signal at the interception line.

[0031] 2025-11-14 BD-G0121-24 WO - Description: The internal correlation device can be implemented like a computer, as described in more detail elsewhere. In particular, the correlation device can be equipped with a computer program product configured for statistical evaluation or error correction of bit sequences. For example, the internal correlation device can be designed to first store a sequence of the measured output probe qubits and then, taking into account the signal captured at the intercept line, to estimate the QKD key possibly negotiated between sender A and receiver B.

[0032] Verification assembly according to one of the preceding claims, further comprising: A communication line which is intended to communicate the estimated QKD key externally and to receive request data from externally in order to control a variation of the parameterization of the quantum computer via the communication line.

[0033] In other words, the QKD key estimated by the verification assembly is communicated externally. "External" could refer, for example, to a laboratory environment, where the laboratory might be dedicated to verifying quantum protocols. Furthermore, the communication link can be bidirectional. Control signals can be sent from the external location to the verification assembly via this communication link, redefining the quantum computer's parameters. For example, a smaller or larger variation of its parameters can be specified, where the parameters could be, for instance, predefined rotation angles of individual gates.

[0034] According to another aspect, an arrangement for verifying a quantum communication protocol is proposed, comprising: a verification assembly as previously described; a transmitter configured to execute the quantum communication protocol to be verified on the sender side; a receiver configured to execute the quantum communication protocol to be verified on the receiver side; and a control device configured to communicate bidirectionally with the transmitter, the verification assembly, and the receiver. In other words, this arrangement provides a laboratory for verifying a quantum communication protocol. The transmitter and receiver are configured to execute the quantum communication protocol to be verified.The control unit is implemented as a computer that manages the transmitter, verification assembly, and receiver, evaluates and assesses the resulting keys, and performs machine learning for parameterizing or modifying the parameters of the internal quantum computer. This parameterization can include optimization according to various criteria, as well as sending the parameters to the internal quantum computer. The arrangement for verifying a quantum communication protocol can therefore be considered a monitoring instance, specifically a monitoring instance of a laboratory for verifying quantum protocols.

[0035] 2025-11-14 BD-G0121-24 WO - Description It should also be understood that the arrangement for verifying a quantum communication protocol does not necessarily have to be embodied – rather, providing a computer program product that simulates the functionalities of the arrangement for verifying a quantum communication protocol is also being considered. In this aspect, all communication lines are then naturally not embodied in the form of lines or wires, but rather in the form of interfaces.

[0036] In embodiments, an arrangement for verifying a quantum communication protocol is proposed, wherein the control device further comprises: a transmitter control line between the transmitter and the control device, the transmitter implementing a quantum protocol to be verified; an input line for monitoring the public communication channel, in communicative contact with the connection point of the verification assembly, which connection point is communicatively connected to the public communication channel; a transmit and receive line between the verification assembly and the control device, which transmit and receive line is in communicative contact with a connection point of the verification assembly, thus enabling bidirectional communication between the control device and the verification assembly;Receiver control line between the receiver and the control device, which receiver control line enables a communicative connection between receiver and control device.

[0037] The transmitter control line connects the control unit to the transmitter. From the perspective of a laboratory for verification purposes, the control unit can be viewed analogously to a laboratory console from which the experimental setup is controlled, the experimental results are recorded and processed, and the parameters of the internal quantum computer within the verification assembly are trained and / or optimized. In some embodiments, the control unit may send the signal to start the verification to the transmitter via the transmitter control line. The transmitter and receiver implement the quantum protocol to be verified. For example, the transmitter and / or receiver may be commercial products that are to be subjected to verification, either together or individually.Via the input line for monitoring the public communications signal, the control device may be able to monitor and / or document the key negotiation between the sender and the receiver. As used herein, the term "listening line" may be equivalent to the term "public communications channel."

[0038] A transmit and receive line can be provided between the verification assembly and the control unit. This transmit and receive line allows the control unit to obtain the current status and results of the verification assembly, particularly relevant for the operation of a laboratory for the verification of a

[0039] 2025-11-14 BD-G0121-24 WO - Description of the quantum communication protocol, the current raw key, and the possible production key of the eavesdropping party represented by the verification assembly. Furthermore, the transmit and receive line can be configured to receive an updated parameterization of the internal quantum computer from the control unit in the verification assembly. More precisely, the transmit and receive line can be internally and directly communicatively connected to an internal control unit of the verification assembly. In embodiments, the internal control unit can be configured like a computer, as described in more detail elsewhere in this application. The internal control unit can be configured to convert the received parameters for the internal quantum computer and feed them into the quantum computer.Furthermore, the internal control unit can be configured to implement the functionality of the verification assembly with regard to its control. In particular, the internal control unit can be configured to control the internal components, attempt to replicate the external protocol, perform statistics and error handling, determine estimates for a key, especially the coordination of the internal qubit generator with the internal qubit sensor, and control the internal quantum computer.

[0040] In some embodiments, a receiver control line may be provided between the receiver and the control device. This receiver control line can provide a communication link between the receiver and the control device.

[0041] In this way, a laboratory for verifying a quantum protocol can be provided, with a control device for controlling the experimental procedure, in particular for controlling a pair of transmitter and receiver in which the quantum protocol to be verified is implemented, for recording and processing the results, and for controlling an eavesdropping verification device, including optimization, for example by machine learning, of the parameters of an internal quantum computer used therein.

[0042] According to a further aspect, a method for verifying a QKD protocol under test is proposed, comprising: providing an arrangement for verifying a quantum communication protocol; executing the QKD protocol under test between the sender and the receiver with the verification assembly interposed communicatively in the quantum channel, wherein a sender-side qubit in a quantum computer is entangled in the verification assembly such that the sender-side qubit is forwarded to the receiver and the resulting entangled qubit within the verification assembly is stored; determining a receiver-side raw key, KEY-B, and its quality, QBER, as well as a verification assembly-side raw key, KEY-E, or production key KEY-E;Determine a receiver-side user key, KEY-B-USE, production key KEY-B and a verification assembly-internal user key, KEY-E-USE; discard the QKD protocol to be tested if the receiver-side user key, KEY-B-USE, and the;

[0043] 2025-11-14 BD-G0121-24 WO - Description: The verification module's internal user keys, KEY-E-USE, are identical. It is clear that the receiver-side raw keys or user keys KEY-B and KEY-B-USE are equivalent to the sender-side raw keys or user keys KEY-A and KEY-A-USE.

[0044] The procedure involves controlling the participating components. Specifically, a sender-side raw key KEY-A, a receiver-side raw key KEY-B, and an internal verification module raw key KEY-E can be determined in the sender. Based on a comparison via the publicly accessible channel, it can be provided that each sender, receiver, and verification module determines a QBER (Quick Key Requirement). Using this result, and optionally incorporating cascading, for example, by using block-wise determined parities, a sender-side user key KEY-A-USE or production key KEY-A, a receiver-side user key KEY-B-USE or production key KEY-B, and / or an internal verification module user key KEY-E-USE or production key KEY-E can be determined. It is understood that this can only be determined under laboratory conditions.If the verification module's internal key KEY-E-USE matches the sender's key KEY-A-USE and / or the receiver's key KEY-B-USE, the quantum protocol can be considered solved or, colloquially, cracked. However, if the key quality of the key negotiated between sender A and receiver B is unacceptably poor, it can be assumed that either the transmission path is inadequate or that the verification module's eavesdropping attempts have disrupted the communication too severely, rendering such an attempt unusable. Therefore, in the quantum protocol verification laboratory described above, these two conflicting factors must be balanced and evaluated.

[0045] The quantum protocol to be verified can also include a simulation thereof, depending on the embodiment. In particular, the protocol can also be implemented entirely in software. Furthermore, each of the components described above can be implemented as software. A quantum communication protocol can be understood as hardware and / or software and / or hardware with corresponding firmware.

[0046] In other words, the present application begins with the realization that no verification logic exists for the security of the implementation of quantum communication protocols, so a task was undertaken to provide a verification logic for the security of the implementation of quantum communication protocols.

[0047] Verification is based on measuring the resilience of the quantum communication protocol implementation to explicit attacks. These attacks are carried out via gate operations on a quantum computing device, such as the internal quantum computer, which is coupled to the quantum communication channel. The quantum states of the communication protocols are further processed on the quantum computing device. This includes the following processing steps: Finding the

[0048] 2025-11-14 BD-G0121-24 WO - Description: Attacks on the protocol using variational quantum circuits. 2) Using the learned attacks found in (1) to execute the protocol under technically relevant attack conditions. 3) Collecting data between the communication partners and the attacker's corresponding data on the distributed quantum states. 4) Applying possible classical digital post-processing to the collected data from (3). This generates a final data set that corresponds to the execution of the quantum communication protocol in reality. 5) Investigating the final data set, which means: - Measuring the probability of success for the attacker, Eve, E, to obtain the same information as the communication partners, namely sender, Alice, A, and receiver, Bob, B. From this, a resistance profile for the quantum communication protocol is derived.A possible comparison of the experimentally determined resistance profile with a predefined, e.g., external, required safety threshold can lead to a "pass" or "fail" assessment. The goal is to identify potential and unknown vulnerabilities in quantum communication protocols.

[0049] One advantage is the ability to verify a quantum communication protocol using real hardware instead of mathematical security proofs. "Real hardware" here also means that a computer program, or a combination of one or more computer programs with hardware components, can simulate real hardware.

[0050] In short, quantum computing devices can analyze the quantum states of quantum communication protocols to identify potential and unknown vulnerabilities in the protocols.

[0051] 2025-11-14 BD-G0121-24 WO - Description Brief description of the figures

[0052] Figure 1 shows a verification assembly for verifying a quantum communication protocol;

[0053] Figure 2 shows an arrangement for verifying and / or validating a quantum communication protocol;

[0054] Figures 3A,

[0055] Figures 3B and 3C show exemplary embodiments of a quantum computer in circuit form, or a quantum computing device or a variational quantum circuit, that can be used in the verification assembly;

[0056] Figures 4A and 4B show a flowchart of a procedure that can be used in an arrangement for the verification and / or validation of a quantum communication protocol;

[0057] Figure 5 shows, as an example of an error handling procedure, a cascading procedure based on parity checking;

[0058] Figure 6 shows a plot of different error rates in an attempt by an eavesdropping party between the sender and the receiver;

[0059] Figure 7 shows, from the background, an arrangement for carrying out a quantum protocol, here exemplified by an indicated eavesdropping party in the quantum channel;

[0060] Figure 8 shows, from the background, a flowchart of a procedure for carrying out a quantum key exchange (QKD) or a procedure for setting up quantum communication;

[0061] Figure 9 shows a block diagram of an exemplary computer system 902 for implementing the considerations presented herein. The computer system may, for example, include a server, a desktop computer, a laptop, a tablet, or a mobile device; and

[0062] Figure 10 shows a block diagram of a Quantum Information Processing System 1000 with an exemplary Quantum Processing Unit or quantum computer and interactions with the previously described classical computer system 902, using direct communication or communication via a Quantum Cloud Service Provider (QCSP).

[0063] Detailed description

[0064] The following description details the individual components, such as the verification assembly with internal quantum computer, control unit, transmitter, and receiver, as embodied devices. It should be understood, of course, that the functionalities of these

[0065] 2025-11-14 BD-G0121-24 WO - Description Components may alternatively be provided as computer program products, so that all of the functionality presented herein can be simulated in part or in its entirety.

[0066] Figure 1 shows a verification assembly 100 for verifying a quantum communication protocol, which verification assembly can also be referred to as an apparatus-like, eavesdropping party E. The verification assembly 100 has an input 110 for receiving a qubit / quantum (input qubit), also referred to as a quantum channel input 110. The qubit received at the input is forwarded via a quantum channel 130, or the forwarding quantum channel 130, where it can be processed in a quantum computer 105, a quantum computing device 105, or a variational quantum circuit 105, or incorporated into a quantum calculation. The quantum computer 105 or variational quantum circuit 105 can be configured to be externally parameterizable. Furthermore, the quantum computer 105 can be designed like a Quantum Processing Unit 1002 described with reference to Figure 10.The functionality of the computer system designated with reference numeral 902 in Figure 10, as described in this explanation of Figure 1, is provided by the correlation device 111. The functionality of the internal control unit 145 may, in individual cases, be coupled or coordinated with the internal correlation device 111. The digital correlation device 111 may be functionally configured as described in the computer system 902 with reference to Figure 9.

[0067] A control line 101 extends from the digital correlation device 111 to digitally control a qubit-generating internal qubit generator 103, which leads via a quantum channel 104 to the quantum computer 105 (Q). In embodiments, the quantum channel 104 can be configured as an input entanglement quantum channel 104. The quantum computer 105 serves, ideally, to copy a qubit received via input 110 as accurately as possible and to store the copy internally, for example in the correlation device 111 or in the internal control unit 145, for later use, with the qubit received at input 110 being forwarded unchanged to an output 120.While creating a copy of a qubit is not possible in principle due to the No Cloning Theorem, a suitable quantum circuit that provides entanglement with a controlled, predefined internal qubit and subsequent measurement of the internal result can approximate cloning, potentially leading to the best possible copy through statistical error evaluation and correction.

[0068] Internally connected downstream of the quantum computer 105 is an internal qubit sensor 107 via an output measurement quantum channel 109 or output entanglement quantum channel 109. The measurement result of the internal qubit sensor 107 can be forwarded via a signal line 108 to the internal correlation device 111 for the transmission of the qubit information determined by the internal qubit sensor 107 in digital form.

[0069] An output 120 or quantum channel output 120 is used to output the received qubit from verification module 100 to the outside.

[0070] The verification assembly 100 further comprises a connection point 140 communicatively linked to the correlation device 111 in order to be connected to a public communication channel and to function as a listening device for eavesdropping on the public communication channel. This is necessary when the verification assembly is functionally inserted into a communication link between two communication partners, sender (A) and receiver (B), so that the correlation device 111 is able to attempt to determine a key to be negotiated between sender and receiver, which are connected via a quantum channel comprising the quantum channel 130.

[0071] 2025-11-14 BD-G0121-24 WO - Description The internal control unit 145 is connected via a communication line 147 for communication between the internal correlation device 111 and the internal control unit 145. This serves, for example, the exchange of information between the correlation device 111 and the internal control unit 145, such as the transmission of an intercepted key from the correlation device 111 to the internal control unit 145. Furthermore, the internal control unit 145 is connected to the quantum computer 105 via a communication line 149 for parameterizing the quantum computer 105. The internal control unit 145 can be functionally configured as described in the computer system 902 with reference to Figure 9.Furthermore, a communication line extends from the internal control unit 145, which, via a communicatively attached connection point 150, serves for communication between the internal control unit 145 and an external device 250 (not shown here). The internal control unit 145 can configure the quantum computer 105 via a control line 149.

[0072] Figure 2 shows an arrangement 200 for verifying and / or validating a quantum communication protocol. To begin with an overview, the arrangement 200 comprises a transmitter 210 (A) and a receiver 220 (B), which are communicatively connected via a quantum channel 230 and a public channel 240. The operation of the arrangement, via the transmitter 210 and the receiver 220 with the connecting quantum channel 230 and the public channel 240, can be described as with reference to the arrangement in Figure 7. A verification assembly 100, as described with reference to Figure 1, is quantum-communicatively integrated into the quantum channel 230 and functions as the listening party E. The transmitter 210 and the receiver 220 are controlled by a control device 250. The control device 250 can be functionally configured as described in Figure 9 for computer system 902.

[0073] Transmitter 210 may include a computer (not shown) which may be functionally configured like the computer system 902 described with reference to Figure 9. This internal transmitter computer may, for example, be configured to control and coordinate the qubits and classical negotiation signals to be transmitted via the public channel, as well as to communicate with a user and / or the control device 250.

[0074] For example, the receiver 220 may include a computer (not shown) which may be functionally configured like the computer system 902 described with reference to Figure 9. This receiver-internal computer may, for example, be configured to control and coordinate the received qubits and classical negotiation signals via the public channel, as well as to communicate with a user and / or the control device 250.

[0075] Specifically, to verify and / or validate a quantum communication protocol, the control unit 250 is communicatively connected to the transmitter 210 via a transmitter control line or control and data line 261. The transmitter 210 has a randomly generated bit and / or qubit generator 215. A randomly generated modifier 217 of the generated qubits is connected downstream of the randomly generated qubit generator 215 – for example, the randomly generated modifier 217 can randomly select a base. The qubit thus processed can be fed directly into the verification module 100 with the quantum channel 130 via a quantum channel input 110. It is also conceivable that a quantum channel 230 is provided between transmitter 210 and receiver 220, into which the verification assembly 100 with the internal quantum channel 130 is communicatively interposed.

[0076] 2025-11-14 BD-G0121-24 WO - Description: The qubit emitted by transmitter 210 is processed in the verification unit 100 by the quantum computer 105 provided therein and finally forwarded to receiver 220 via the output 120 of the verification unit 100. A randomly controlled modifier 227 is provided at the input of receiver 220, which, for example, randomly selects a basis after which the qubit is measured in the measuring device 225 of receiver 220.

[0077] The control unit 250 has a transmit control line 261 or a control and data line 261 for controlling the transmitter 210 and receiving data from the transmitter 210. Furthermore, the control unit 250 has a receiver control line 267 or a control and data line 267 for controlling the receiver 220 and receiving data from the receiver 220. The control unit 250 also has an input line 263 for monitoring the public communication channel 240, in communicative contact with the connection point 140 of the verification module 100. Finally, the control unit 250 has a transmit and receive line 265 between the verification module 100 and the control unit 250, in communicative contact with the connection point 150 of the verification module 100.

[0078] In summary, the arrangement 200 can be used to verify a quantum communication protocol: For example, the transmitter 210 and the receiver 220 operate according to a communication protocol to be verified and attempt to determine a key via the quantum communication channel 230 and the public channel 240. A verification assembly 100 is connected to the quantum communication channel 230. The control unit 250 serves not only to control the transmitter 210 and receiver 220, but also to acquire and monitor the results from the transmitter 210 and receiver 220, as well as to control and monitor the verification assembly 100, which assumes the function of an eavesdropping party.The control unit 250 is further equipped to perform the statistical calculations and evaluations of the resulting keys between transmitter 210 and receiver 220 as well as in the verification assembly 100 and to optimize the parameterization of the quantum computer 105 based on the recorded statistics with regard to suitable metrics.

[0079] Figures 3A, 3B, and 3C show different exemplary variations of a quantum computer 105 in circuit form, or a quantum computing device 105, or a variational quantum circuit 105, which can be used in embodiments of the verification assembly 100. The different variations of the quantum computer 105 each have a quantum channel input 110, which, for the sake of simplicity, is considered equivalent to the quantum channel input 110 of the higher-level verification assembly 100. A corresponding quantum channel output 120 is also shown. Similarly, an input entanglement quantum channel 104 and an output entanglement quantum channel 109 are shown.At this point, and representing other passages herein, it should be noted that the terms input entanglement quantum channel and output entanglement quantum channel are not intended to exclusively imply that these channels are solely for sending or outputting a qubit to be entangled or an entangled qubit. However, embodiments are conceivable in which the entanglement of the internal qubit in the verification assembly 100 with the qubit of the key exchange path between A and B, which is fed via the quantum channel input 110, plays an important role.

[0080] The parameters of the individual quantum gates in the quantum computer 105 can be externally parameterized in order to be optimized in a higher-level process, such as QCL (Quantum Circuit Learning). "Optimal" can be understood, for example, as meaning that the qubit or key negotiation qubit in the key negotiation path 230, or quantum channel 230, or the contained quantum channel 130, from sender A to receiver B, is as close as possible, possibly statistically so, to the qubit's maximum extent.

[0081] 2025-11 -14 BD-G0121 -24 WO - Description arrives unchanged at recipient B and simultaneously the internal qubit of the verification assembly 100 is entangled with the key negotiation qubit in such a way that a copy of the key negotiation qubit that is as similar as possible can be stored in the verification assembly 100, i.e. the eavesdropping party E, for the later purpose of key reconstruction for eavesdropping purposes.

[0082] Figure 3A shows a quantum circuit 105 or quantum computer 105 with a plurality of rotary gates and two CNOT gates, wherein, in this example, the angles of the rotary gates can each be individually parameterized from the outside.

[0083] Figure 3B shows a quantum circuit 105 for an “Imbalanced Cloner”, with a plurality of rotary gates with a predefined rotation angle, two CNOT gates and two externally parameterizable rotary gates, namely in this example R z (i ) and R z (,(p). Due to the two free parameters i and < >, two degrees of freedom can be adapted.

[0084] Figure 3C shows a quantum circuit 105, also known as a PCCM (Phase Covariant Cloning Machine). It contains a few rotary gates with a predefined angle, no CNOT (Combined Node Target Clock), and one rotary gate with an externally configurable rotary gate R. y(0) is provided, which here operates as a controlled rotary gate. With a specific choice of the angles i and < > of the circuit in Figure 3B, this gate can be considered a special case of an "Imbalanced Cloner". Crucially, this is a 2-qubit gate capable of generating entanglement between the two qubits. This can also be referred to as an SU(4) gate (Special Unitary Group of Order 4), which must generate entanglement between two qubits.

[0085] Figures 4A and 4B show a flowchart of a procedure that can be used in an arrangement for verifying and / or validating a quantum communication protocol. The procedure 400 for verifying a quantum communication protocol begins in a "Start" step 410. In an initialization step 415, transmitter 210, receiver 220, and verification assembly 100 are initialized, whereby the parameters for the quantum computer 105 can also be set in the verification assembly 100. In a step 420, the transmitter 210 is started. The aforementioned steps 410, 415, and 420 are executed in the control unit 250, which is implemented as computer 250 according to a computer 902 described with reference to Figure 9.In step 425, the transmitter 210 sends at least one randomly generated bit and / or qubit, or a qubit modified by a randomly controlled transmit modifier 217 or a randomly controlled transmit base filter 217, via a quantum channel 230 towards the receiver 220. In some embodiments, only a single qubit may be sent at a time. In alternative embodiments, a plurality of qubits may be sent sequentially. The number of qubits sent may be predetermined or adjusted according to the results of a statistical evaluation.

[0086] In step 430, the verification assembly 100, using the quantum computer 105 provided in the verification assembly 100, entangles each of the qubits and forwards the entangled qubit received from the sender 210 to the receiver 220, storing a qubit referred to here as a clone candidate qubit in the verification assembly 100. The term "clone candidate" reflects the fact that an exact clone of a qubit cannot, in principle, be created; only a "best trial" attempt can be made.

[0087] In step 435, the receiver 220 measures the received qubit, modified by a randomly controlled receive modifier 227 or receive base filter 227, using measuring device 225 and stores it in the receiver 220. In step 440, transmitter 210 and receiver 220 compare via

[0088] 2025-11 -14 BD-G0121 -24 WO - Description of a public channel 240, the transmitter-side raw key KEY-A with the receiver-side raw key KEY-B, which is done by exchanging the transmit modifier settings and receiver modifier settings and can be described as "sifting".

[0089] In step 445, the verification assembly 100 or the listening party, E, listens to the comparison and creates its own raw key KEY-E. In step 450, sender 210 and receiver 220 perform a quantum bit error rate (QBER) estimation. In step 460, error comparison and / or key correction may be performed between sender 210 and receiver 220. This will be done by a computer located in sender 210 (not shown here) and a computer located in receiver 220 (also not shown here) via public channel 240. Both sender-side and receiver-side computers can be independently configured as described in detail with reference to Figure 9 and the associated description of computer 902.

[0090] Based on the evaluated raw key, and if necessary after appropriate error correction or stripping of bits, a production key can be determined for each participating party, which from the perspective of the respective party is then referred to as production key KEY-A or production KEY-A or production key KEY-A for party A, and for parties B and E accordingly as production key KEY-B and production key KEY-E.

[0091] A person skilled in the art understands that the verification module 100, E, also performs its own error correction and / or key correction by accessing the publicly available channel, based on the information exchanged between sender, A, and receiver, B. In this way, the verification module can also obtain a production key KEY-E.

[0092] The connection points A and B between figures 4A and 4B are considered self-explanatory.

[0093] In step 465, the receiver 220 sends the determined value QBER and the production key (KEY-B), determined by means of, among other things, suitable error handling, to the control device 250. In step 475, the results are evaluated and corresponding branching takes place. A person skilled in the art understands that such a step is not possible in a scenario of actual communication between two parties A and B. Rather, such a step is only possible in a verification scenario, which can also be considered executing the step under laboratory conditions. Ultimately, embodiments involve the verification of a quantum protocol and thus, ultimately, its evaluation in the sense of quality assurance with regard to the cryptographic security of an established or to-be-established quantum communication link.

[0094] The expert understands that the user key production key KEY-B must be equal to the user key production key KEY-A for communication between A and B to occur. The protocol is then successfully executed. Only when the product keys KEY-A and KEY-B are identical does a comparison with product key KEY-E make sense.

[0095] If, as is checked in embodiments in the control device 250, the condition Productive Key E == Productive Key B is established, the eavesdropping party E has complete knowledge of the communication key negotiated between parties A and B regarding the quantum protocol QKD under investigation. In other words, the quantum protocol can be considered to be broken. In this case, it may be appropriate to terminate the procedure by issuing appropriate information, for example, a message of the type "QKD protocol insecure," in step 486.

[0096] 2025-11-14 BD-G0121-24 WO - Description In embodiments, consideration is given to providing a global termination criterion. Therefore, step 485 shows that when the global termination criterion is met: termination by the control device 250, with output of the reason for the termination. The global termination criterion can be met, for example, when a measured runtime is greater than or equal to a predetermined maximum runtime. Alternatively, a maximum number of loop iterations can be predetermined, upon reaching or exceeding which the procedure is terminated at that point. Alternatively, the global termination criterion can be enforced by pressing a predetermined key on the control device 250, which is implemented like a computer 902, for example, a <esc>-Taste.

[0097] In step 490, the procedure is continued in the control unit 250 if the evaluation performed by this unit determines that the error rate QBER < CQBERIQKD, meaning that in a real-world scenario, parties A (sender 210) and B (receiver 220) would consider the quality acceptable and commence communication using the determined payload key. This occurs when the error rate is lower than the critical error rate specified for the selected and verified QKD protocol. The parameters sent to the quantum computer 105 of the verification assembly 100, which led to this evaluation result, are stored separately in the control unit 250. This achieves a partial goal of the overall setup: that parties A and B, simply put, do not realize they have been eavesdropped on.Based on this parameterization, it might then only be necessary, for example, to optimize the parameters, possibly in small increments, so that the key determined by the eavesdropping party is as identical as possible to the key that A and B jointly determined. Accordingly, in a subsequent step 491, the parameters for configuring the quantum computer 105 can be varied by one optimization step in the control unit 250. Furthermore, the parameters can be stored in a separate memory table in the control unit 205 as "A and B noticed nothing." Continuing the procedure despite achieving the partial goal can be useful in order to train the parameters for achieving this goal for later purposes. The optimization step can be carried out in a known manner using a gradient descent method to find locally nearby new parameters.

[0098] If the evaluation determines that the error rate QBER > CQBERIQKD, such that in a real-world scenario parties A (sender 210) and B (receiver 220) would consider the quality unacceptable and attempt a new key exchange, the procedure continues in step 495. In a subsequent step 496, the parameters for configuring the quantum computer 105 are varied by one search step in the control unit 250. This search step differs from the previously described optimization step in that the parameters to be set are selected at a significantly greater distance from the current parameter set than would be the case in the optimization step, since a coarser search may be required in this case. The currently tested parameters can be stored in a memory table in the control unit 250 as "A and B have detected an error".The search step can be determined using a gradient method with a larger local distance or by calculating random new parameters within a local sphere around the point of the current parameters.

[0099] In a subsequent step 497, the parameters newly set in step 491 or step 496 are sent by the control unit 250 to the verification unit 100, so that the latter sets the new parameters in the quantum computer 105. The process continues in step 415.

[0100] The process ends in step 480.

[0101] 2025-11-14 BD-G0121-24 WO - Description Figure 5 shows, as an example of an error handling procedure for determining a common key, a cascading procedure based on parity checking: The disadvantage of the usual procedure known from BB84 is the sacrifice of individual bits to calculate the quality of the determined key based on the error rate OBER, and thus to be able to perform the comparison (QBER < CQBERIQKD) for the protocol QKD to be verified. Therefore, a block-wise comparison of parities is proposed in order to identify errors in this way, and perhaps even to repair them (error reconciliation), without having to sacrifice too many bits.

[0102] Figure 500, illustrating a cascading error correction process as proposed in embodiments, for example, after successful sifting, shows in reference numeral 510 a comparison of sender-side and receiver-side bit groups as parts of the raw key, with cascading to locate the erroneous bit in the event of a parity mismatch. In this example, the sender, A, determines a parity "0" for an example block, and the receiver, B, determines a parity "1" for the key segment associated with this example block. This is communicated in a grouped manner, as shown in Figure 520. The sender-side and receiver-side blocks are then halved, communicated again in a grouped manner, and the parities are compared for each halved block, so that the error can ultimately be isolated and then eliminated without exchanging the bit contents over the public channel.The eavesdropping party, E, has access to the information exemplified in 530; however, due to the grouped representation without content, E cannot reconstruct any key components. Finally, in 540, B is able to correct the error. As a result, a comparatively much larger proportion of the transmitted bits can be used for the key.

[0103] Figure 6 shows a plot of 600 different error rates during an interception attempt by an eavesdropping party between sender A and receiver. The x-axis (610) represents the error rate of the raw key during the key exchange between party A and party B, i.e., before cascading (i.e., before error correction, caused, among other things, by eavesdropping by party E). The y-axis (620) represents the fidelity of the eavesdropping party, E. The fidelity, plotted as a percentage, represents the proportion of key bits correctly determined during the sifting process between A and B and, of course, requires comparison with the actual key. If the fidelity is, for example, 100%, the key has been determined, or "cracked," by the eavesdropping party, and the QKD to be verified would have to be considered insecure.Reference numeral 630 denotes an approximation curve over a large number of runs of the relative proportion of correctly guessed keys compared to the error rate of the raw key.

[0104] Figure 9 shows a block diagram of an exemplary computer system 902 for implementing the considerations presented herein. The computer system can, for example, include a server, a desktop computer, a laptop, a tablet, or a mobile device.

[0105] The components of the 902 computer system may include, but are not limited to, one or more processors or processing units (CPUs) (903), a storage system (911), a storage unit (905), and a bus (907) that connects various system components, including the storage unit (905), to the processor (903). The storage system (911) may, for example, include a hard disk drive (HDD). The storage unit (905) may include computer-readable media in the form of volatile memory, such as RAM and / or cache memory.

[0106] The 902 computer system can also communicate with one or more external devices, such as a keyboard, a pointing device, a 913 display, etc., or with an external device 913 if the computer system is designed as a so-called embedded device; for example, one or more devices that allow an operator to communicate with the 902 computer system; for example, any device (network card,

[0107] 2025-11-14 BD-G0121-24 WO - Description (Modem, etc.), which enable the Computer System 902 to communicate with one or more computer devices. Such communication can take place via I / O interface(s) 919. Furthermore, the Computer System 902 can communicate with one or more networks, such as a local area network (LAN), a wide area network (WAN), and / or a public network, such as the Internet, via a Network Adapter 909. As shown, the Network Adapter 909 communicates with the other components of the System 902 via the Bus 907.

[0108] The 905 memory unit is designed to store applications in the form of instructions that can be executed on the 903 processor. For example, the 905 memory unit can contain an operating system and one or more application programs.

[0109] As understood by those skilled in the art, aspects of the present invention may be implemented as an apparatus, device, kit, assembly, method, computer program, or computer program product, or as an arrangement composed of several of the aforementioned entities interconnected in communication with one another. Accordingly, aspects of the present invention may take the form of an embodiment implemented entirely in hardware, an embodiment implemented entirely in software (including firmware, resident software, microcode, etc.), or an embodiment implemented as a combination of software and hardware aspects, all of which are hereby generally referred to as a "circuit," "module," or "system." Furthermore, aspects of the present invention may take the form of a computer program product implemented on one or more computer-readable media and embodying computer-executable code therein.A computer program comprises the code or “program instructions” that can be executed by a computer.

[0110] The term "computer system" refers to data processing hardware and encompasses all types of apparatus, devices, equipment, kits, assemblies, modules, and machines for processing data, including a programmable processor, a computer, or multiple processors or computers. The apparatus may also consist of or include: specialized logic circuits, a central processing unit (CPU), an FPGA (field-programmable array), or an ASIC (application-specific integrated circuit). In some implementations, the data processing apparatus and / or the specialized logic circuitry may be hardware-based or software-based. The apparatus may optionally include code that creates an execution environment for a computer program, such as code representing processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of these.The present disclosure considers the use of data processing equipment with or without a conventional operating system, such as LINUX, UNIX, WINDOWS, MAC OS, ANDROID, IOS, RASPBERRY or any other suitable conventional operating system.

[0111] Any combination of one or more computer-readable media can be used. The computer-readable medium can be a computer-readable storage medium. The term "computer-readable storage medium" as used herein includes any physical or tangible storage medium capable of storing instructions executable by a computer device's processor. The computer-readable storage medium may also be referred to as a tangible computer-readable medium. In some embodiments, a computer-readable storage medium may also be capable of storing data accessible by the computer device's processor.

[0112] "Computer memory" or "memory" is an example of a computer-readable storage medium. Computer memory is any memory that can be directly accessed by a processor.

[0113] 2025-11-14 BD-G0121-24 WO - Description can. Besides program instructions, the memory can also contain, for example, parameters to be optimized for parameterizing a quantum computer, which are sent via an interface from the computer system 902 to a quantum processor unit 1002, as explained in more detail below.

[0114] The term "processor," as used herein, encompasses an electronic component capable of executing a program, machine-executable instructions, or computer-executable code. References to computer equipment that includes a "processor" should be understood to mean that it may contain more than one processor or processor core. For example, the processor may be a multi-core processor. The term "processor" may also refer to a group of processors within a single computer system or distributed among multiple computer systems. The computer-executable code may be executed by multiple processors located within the same computer equipment or even distributed across different computer equipment.

[0115] Computer-executable code can comprise machine-executable instructions or a program that causes a processor to execute an aspect of the present invention. Computer-executable code for performing operations relating to one or more aspects of the present invention can be written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Java, Smalltalk, C++, or the like, as well as conventional functional programming languages ​​such as the programming language "C," and compiled into machine-executable instructions. In embodiments, the computer-executable code can be in the form of a high-level language or in pre-compiled form and used in conjunction with an interpreter that generates the machine-executable instructions "on-the-fly."

[0116] In general, program instructions can be executed on one or more processors. In the case of multiple processors, these can be distributed across several units. Each processor could execute a portion of the instructions intended for that unit. Therefore, when referring to a system or process that comprises multiple units, the computer program or program instructions should be understood as being adapted to be executed by a processor associated with or assigned to the respective unit.

[0117] Figure 10 shows a block diagram of an exemplary Quantum Processing Unit or quantum computer and interactions with the previously described classical computer system 902, using direct communication or communication via a Quantum Cloud Service Provider (QCSP).

[0118] In one embodiment, a classical computer system 902, which may be configured like the computer system described above, is directly connected or coupled communicatively with a quantum processing unit (QPU) 1002 of a quantum information processing system (QIPS) 1000, wherein the computer system 902 is configured to send instructions for the execution of quantum operations to the QPU 1002.

[0119] The QPU 1002 can be configured in embodiments as a quantum computer, a quantum computing device, and / or a variational quantum circuit. In particular, it can be provided that the QPU 1002 is externally parameterizable using classical methods. For example, it is considered that the angle of a single quantum circuit element can be externally adjusted using classical methods.

[0120] 2025-11-14 BD-G0121-24 WO - Description The instructions sent by the Computer System 902 may include, but are not limited to: instructions for one or more quantum gates and / or quantum operations to be applied to one or more qubits on the QPU; instructions for transpilation and / or optimization of a quantum circuit or quantum computation to be performed on the QPU 1002, for example, instructions for error overcoming, error correction, and / or optimal mapping of logical qubits to physical qubits on the QPU 1002; and / or instructions for a number of measurements to be performed on the QPU 1002.The instructions can also include default values ​​for parameters for individual quantum circuits, so that a given quantum gate can be flexibly parameterized, for example, as a Pauli matrix for a predefined axis or, in a subsequent parameterization, which was determined, for example, in an optimization step, as an R-gate with a predefined angle for a rotation about a predefined axis.

[0121] The instructions received by the computer system 902 from the QPU 1002 after the execution of a quantum circuit may include, but are not limited to: measurement results, information about the runtime of the executed quantum circuit or quantum computation, the quantum gates or quantum operations performed, and / or the order in which the quantum gates or quantum operations were executed. In one considered embodiment, measurement results are provided to the classical computer 902 as a dictionary, wherein keys of the dictionary correspond to basis states of the quantum system on the QPU 1002, and values ​​of the dictionary correspond to a number of measurements of the basis state of the corresponding key of the dictionary. The keys may, for example, be provided as bit strings, with each bit corresponding to the state in which a particular qubit was measured.

[0122] In an alternative embodiment, the (classical) computer system 902 can be communicatively coupled or connected to a Quantum Cloud Service Provider (QCSP) 1004, wherein the QCSP 1004 is in turn communicatively connected to the QPU 1002. In this alternative embodiment, the instructions sent by the computer system 902 to the QPU 1002 and received by the computer system 902 from the QPU 1002 are instead sent to and received by the QCSP 1004, respectively.

[0123] User 1004 can access computer system 902, for example, via computer system 902's I / O interface 919. User 1004 can input commands or instructions and receive responses from computer system 902. The commands or instructions can be directed to computer system 902 (the traditional system) or Ql PS 1000, specifically QPU 1002 and / or Quantum Cloud Service Provider QCSP 1004. The responses can be received by computer system 902 (the traditional system) or Ql PS 1000, specifically QPU 1002 and / or Quantum Cloud Service Provider 1004.

[0124] Qubits can be in states that are basis states of the quantum system, where the basis states are denoted as 10> and 1 1> and are analogous to the states 0 and 1 of a classical bit. Qubits can also be in states that are superpositions of several basis states of the quantum system, where each basis state is associated with an amplitude, and the amplitude is associated with a probability of measuring the qubit in the basis state associated with that amplitude. For example, given a sufficiently large number of measurements, the superposition state t ) = |0) + ^3 / 411) as

[0125] | 0> in 25% of measurements and as 1 1> in 75% of measurements. Amplitudes of basis states can be updated or in a quantum computation consisting of a set of quantum gates in a quantum circuit or a set of quantum operations.

[0126] 2025-11-14 BD-G0121-24 WO - Description changed. The process, which includes the initialization and execution of a quantum calculation, with subsequent measurement, can be referred to as the "shot" or "run" of such a quantum calculation.

[0127] Individual qubit quantum gates include, for example, the X, Y, and Z Pauli gates, and the rotary gates R x (0), Ry(0), Rz(ß) , which are parameterized by an angle 6, the H-gate, also known as the Hadamard gate, and the Phase shift gate parameterized by an angle < >.

[0128] Entanglement between qubits can be achieved by executing multi-qubit gates. A multi-qubit gate can include, for example (but is not limited to), the SWAP gate, which swaps the states of two qubits, or controlled gates, such as the CX gate, also known as the CNOT gate. Controlled gates can have any number of control qubits and one target qubit. A quantum operation is performed on the target qubit when all control qubits are in the state 1. For example, the CCX gate includes two control qubits that act as an X gate on the target qubit only when both control qubits are in the state 1.

[0129] The QPU 1002 can be restricted to the execution of a specific set of gates, referred to here as natively available gates. Any quantum circuit can then be decomposed into a series of gates from this specific set of natively available gates.

[0130] In an alternative embodiment, the QPU 1002 can be a quantum annealer configured to perform a quantum annealing procedure. Quantum annealers can be used, for example, to solve computational problems, particularly problems formulated as combinatorial optimization problems, and especially quadratic unconstrained binary optimization problems (QUBO problems). Solving a QUBO problem involves mapping binary variables or features to the states of qubits and / or constraints of these variables, or mapping features to interactions between the qubits. The measurement of the qubits used to solve the QUBO problem results in a solution in the form of a binary bit string, where each bit corresponds to a state of the qubit and thus to the value of the associated binary variable.The solution ideally corresponds to the ground state of the quantum system encompassed by the qubits of the quantum annealer.

[0131] In another embodiment, optimization problems can be solved using a gate-based quantum computer such as the QPU 1002 using a Quantum Approximate Optimization Algorithm (QAOA).

[0132] The implementation of the states 10> and 1 1> of a qubit, the structure of operations to be applied to the qubits and the implementation of measurements of qubits may vary depending on the physical quantum system used in the QPU 1002.

[0133] In one embodiment, for qubits comprising one or more Josephson junctions, wherein a nonlinear inductance enables the formation of discrete energy levels, these energy levels can, for example, be identified with the states 10> and 1 1> of a qubit. In particular, qubits can be implemented as transmon qubits, flux qubits, charge qubits, and / or phase qubits. For example, in a flux qubit, the states 10> and 1 1> correspond to different persistent current states flowing through a superconducting loop. As another example, in a phase qubit, a phase difference across a Josephson junction is used to define the qubit states 10> and 1 1>. The states of the qubit can be manipulated using microwave pulses tuned to the qubit transition frequency.

[0134] 2025-11-14 BD-G0121-24 WO - Description are tuned. The phase, frequency, and / or amplitude of such microwave pulses can be varied to implement different quantum operations. Measurements can be performed, for example, by coupling the superconducting qubit to a resonator, where the resonator frequency shifts depending on the qubit state. The qubit state can thus be detected via the frequency shift by subjecting the resonator to a microwave signal for measurement. Measurements can also be performed by measuring a transmitted or reflected microwave signal from the resonator.

[0135] The use of superconducting Josephson junctions for qubit implementation enables improved scalability because standardized microfabrication methods can be used, resulting in a greater number of qubits in a QPU 1002. Furthermore, the qubits enable fast gate operations on the nanosecond scale.

[0136] In another embodiment, the qubits comprise stored ions, in particular ions stored in an ultra-high vacuum, wherein the ions are stored in space by applying an oscillating electric field, for example as in a quadrupole ion trap (also known as a Paul trap or quadrupole ion trap). In this embodiment, the states 10> and 11> of a qubit can be the ground state and an excited state, or two distinct states of an ion, for example, a ytterbium ion or a calcium ion. The states of the qubit can be manipulated using laser light emitting a frequency tuned to the transition energy between states 10> and 11> to implement quantum operations.For example, a laser emitting a light frequency tuned to the energy difference between the |0> and |11> states can implement a single-qubit rotary gate, where the length of the laser pulse corresponds to the angle of rotation. This laser light can be applied to one qubit or multiple qubits simultaneously to implement single or multiple quantum operations. Measurements can be performed as an optical readout using fluorescence detection. For example, a higher fluorescence signal can correspond to the |10> state, and a lower fluorescence signal can correspond to the |11> state.

[0137] The use of stored ions can be advantageous for qubit implementation, as it allows for longer coherence times compared to other implementations. Furthermore, stored ions can enable full coupling of the qubits in a QPU 1002, which can reduce the number of operations required to implement a quantum algorithm, leading to improved algorithm performance.

[0138] In another embodiment, qubits comprise neutral atoms, particularly neutral atoms stored in optical lattices or optical tweezers. In this embodiment, the states 10> and 11> of a qubit can be the ground state and an excited state, or two different excited states of a neutral atom, for example, two hyperfine states of a rubidium atom. These atoms can further be excited to Rydberg states using laser pulses. The states of the qubit can be manipulated using laser light emitting a frequency tuned to the transition energy between state 10> and 11> to implement quantum operations. Multi-qubit gates can also be implemented using Rydberg blocking, which prevents other neutral atoms from being excited to the same Rydberg state.For example, a laser emitting a frequency tuned to the energy difference between state 10> and state 11> can implement single-qubit rotation gates, where the length of the laser pulse corresponds to the rotation angle. This laser light can be applied to one or more qubits simultaneously to perform single- or multi-qubit quantum operations.

[0139] 2025-11-14 BD-G0121-24 WO - Implement description. Measurements can be performed as an optical readout using fluorescence detection. For example, a higher fluorescence signal can correspond to state 10> and a lower fluorescence signal can correspond to state 1 1>.

[0140] The use of neutral atoms for the implementation of qubits can enable long coherence times, complete connectivity, and manipulation of qubits via standard methods such as optical tweezers, and operation at or near room temperature.

[0141] In another embodiment, qubits can be implemented by nitrogen vacancy centers (NVs) in a solid-state system, particularly in diamonds. This involves replacing a carbon atom in a diamond lattice with a nitrogen atom and creating a vacancy in the lattice immediately adjacent to the nitrogen atom. In this embodiment, states 10> and 1 1> are the spin states of a free electron located in the vacancy of the NV center. The states of the qubit can be manipulated using microwave and optical techniques to implement quantum operations. Measurements can be performed as an optical readout using fluorescence detection. For example, a higher fluorescence signal can correspond to state 10>, and a lower fluorescence signal can correspond to state 1 1>.

[0142] The use of NV centers in diamond for the implementation of qubits enables optical addressability, comprehensive optical initialization, manipulation and readout of qubits, room temperature operation and improved shielding of the qubit from ambient noise by embedding the qubit in the solid lattice of diamond.

[0143] In another embodiment, qubits comprise electronic states of semiconductor quantum dots, for example, silicon quantum dots. The quantum dots can confine electrons or holes, making it possible to use charge or spin states of the electrons or holes as qubits. For example, if a spin state is used, the 10> and 1 1> states of a qubit can correspond to an electron spin of "up" or "down" relative to a reference axis. If, for example, a charge state is used, the 10> and 1 1> states of a qubit can correspond to the presence or absence of an electron in a specific quantum dot, or to the electron being located in one of two coupled quantum dots.The states of the qubit can be manipulated by, but are not limited to, oscillating magnetic and / or electric fields, the use of spin coupling in neighboring quantum dots, and / or the use of electrostatic interaction between charges in neighboring quantum dots. Measurements can be performed, for example, by spin-to-charge conversion methods and / or charge measurement.

[0144] The use of quantum dots for the implementation of qubits enables scaling through the use of standard semiconductor manufacturing processes, particularly in the case of silicon, and allows compatibility with other silicon-based technologies.

[0145] In another embodiment, qubits comprise photons. For example, the 10> and 1 1> states of a qubit can be defined based on photons that have a horizontally or vertically oriented polarization relative to a reference axis. Alternatively, the |0> and 1 1> states can be defined as photons that follow different paths in space. As a further alternative, the 10> and 1 1> states can be defined as photons that arrive at a target, such as a sensor, at an early or late time bin. The states of the qubit can be manipulated using optical elements to implement quantum operations, where the optical elements are waveplates, beam splitters, and / or phase shifters. Linear or nonlinear processes can be implemented in a

[0146] 2025-11-14 BD-G0121-24 WO - Description: Quantum computing and / or operations of a quantum computer, where photons are used as qubits. Measurements can be performed, for example, by detecting photonic polarizations using a polarizing beam splitter and / or waveplate. Alternatively, measurements can be made using interferometric techniques to measure the qubit path, for example, by employing Mach-Zehnder interferometry. As another example, measurements can be performed using time-resolved detection of photons arriving at an early or late time bin, for example, by using single-photon detectors with sufficiently high temporal resolution.

[0147] The use of photons for the implementation of qubits allows for operation at room temperature, high speed of quantum calculations, lower decoherence when transporting information over long distances, and compatibility with standard optical technologies, with special techniques for the creation of photonic circuits.

[0148] In another embodiment, qubits comprise quasiparticles, particularly anyons, embedded in two-dimensional materials. These materials include, for example, topological superconductors, semiconductor nanowires, and / or highly correlated materials. For instance, the qubits can be implemented using a system of Majorana fermions, where pairs of Majorana fermions can be fused to form either a vacuum state representing |0> or a fermion state representing |1>. The states of the qubit can be manipulated by exchanging or entangling anyons to implement quantum operations. Measurements can be performed, for example, by fusion measurements, observing whether a pair of fermions fused into the vacuum state or the fermion state.Linkage measurements can be performed using interferometry techniques or by using auxiliary qubits.

[0149] The use of anyons for the implementation of qubits allows for intrinsic fault tolerance and fault resilience through the topological protection of qubit states.

[0150] It is understood that the invention presented herein may be implemented using a quantum processing unit (QPU) 1002 using an alternative quantum system for the implementation of qubits, which differs from the implementations listed here, for example, if such an alternative quantum system has advantages such as one or more of the following properties (not an exhaustive list): improved noise resistance, longer coherence time, lower gate error rates, lower measurement error rates at larger qubit numbers, and / or advantages in the setup of the hardware of the quantum processing unit (QPU) 1002, for example, with regard to the required cooling of at least parts of the quantum system and its associated hardware, such as sensors or the generation of a vacuum.

[0151] It is further understood that instead of or in addition to qubits, the quantum system used in QPU 1002 can include the use of additional basis states beyond 10 and 11, i.e., the quantum system can include “qudits”, each qudit comprising a number of d basis states with d > 2. For example, a qudit with d=3, a so-called qutit, can assume superpositions of three basis states, denoted as 10, 11 and 12, thus increasing the dimension of the Hilbert space of the quantum system.

[0152] While the invention has been described in detail in the figures and the preceding description, it should be understood that these figures and description are purely illustrative and not limiting. The invention is not limited to the disclosed examples.

[0153] 2025-11 -14 BD-G0121 -24 WO - Description

[0154] 2025-11-14 BD-G0121-24 WO Description Reference List

[0155] 100 verification modules for verifying a quantum communication protocol, eavesdropping party E

[0156] 101 Control line for the digital control of a qubit-generating internal qubit generator 103 by the digital correlation device 111

[0157] 103 Internal Qubit Generator

[0158] 104 Input entanglement quantum channel from internal qubit generator 103 to quantum computer 105 (Q)

[0159] 105 quantum computers (Q) for entangling an internal qubit with the input qubit

[0160] 107 Internal Qubit Sensor

[0161] 108 Signal line for transmitting the qubit information determined by the internal qubit sensor 107 in digital form to the internal correlation device 111

[0162] 109 Output entanglement quantum channel from quantum computer 105 to internal qubit sensor 107

[0163] 110 Input for receiving a qubit / quantum or input qubit at verification module 100, quantum channel input 110

[0164] 111 Internal Correlation Device

[0165] 120 Output for outputting a qubit / quantum or output qubit) from verification module 100, quantum channel output 120

[0166] 130 Quantum channel of the verification assembly 100 for forwarding the input qubit, forwarding quantum channel 130

[0167] 140 Connection point to the wiretapping line for monitoring a public communication channel

[0168] 145 Internal control unit

[0169] 147 Communication line for communication between the internal correlation device 111 and the internal control unit 145

[0170] 149 Communication line for parameterizing the quantum computer 105 (Q)

[0171] 150 Connection point of the communication line for communication between the internal control unit 145 and an external facility 250

[0172] 200 Order for the verification / validation of a quantum communication protocol

[0173] 210 Transmitter A

[0174] 215 Random generator of bits / qubits

[0175] 217 Randomly controlled modifier of the generated qubits (e.g. randomly chosen base)

[0176] 220 Receiver B

[0177] 225 Measuring device for measuring the modified received qubit

[0178] 227 Randomly controlled modifier of received qubits

[0179] 230 quantum channel of the arrangement 200 with integrated quantum channel 130 of the verification assembly 100

[0180] 240 Public Communication Channel

[0181] 250 Control device of the arrangement 200

[0182] 261 Transmitter control line or control and data line between transmitter A, 210, and control unit 250

[0183] 263 Input line for monitoring the public communication channel 240, in communicative contact with connection point 140 of the verification module 100

[0184] 265 Transmit and receive line between the verification module 100 and control unit 250, in communicative contact with the connection point 150 of the verification module 100

[0185] 2025-11-14 BD-G0121-24 WO - Description 267 Receiver control line or control and data line between receiver B, 220, and control unit 250

[0186] 400 methods for verifying a quantum communication protocol

[0187] 410 Start

[0188] 415 Initializing transmitter 210, receiver 220 and verification assembly 100, where the parameters for the quantum computer 105 are set in the

[0189] Verification module 100 will also be set

[0190] 420 Starting from channel 210

[0191] Transmitter 210 sends a sequence of qubits modified by random transmit modifier or transmit base filter 217 via quantum channel 230.

[0192] 430 Verification assembly 100 interlocked by means of the in the

[0193] Verification assembly 100 of the quantum computer 105 examines each of the qubits individually and forwards the entangled qubit received from sender 210 to receiver 220 and stores the clone candidate qubit in the verification assembly 100.

[0194] 435 Receiver 220 measures the received qubit, modified by a randomly controlled receive modifier or receive base filter, using measuring device 225 and stores it in receiver 220.

[0195] 440 Transmitter 210 and receiver 220 compare the transmitter's raw key KEY-A with the receiver's raw key KEY-B via public channel 240, by exchanging the transmitter modifier settings and receiver modifier settings.

[0196] 445 Verification module 100 listens to the comparison and creates its own raw key KEY-E

[0197] 450 Transmitter 210 and Receiver 220 perform an error estimate QBER

[0198] 460 [optional: error correction and / or key correction between transmitter 210 and receiver 220]

[0199] 465 Sending, from receiver 220, the determined value QBER and the determined user key KEY-B to the control unit 250

[0200] 475 Evaluation of results and corresponding branching

[0201] 480 End

[0202] 485 When the global termination criterion is reached: Termination with output of the reason for the termination

[0203] 486 If the protocol is broken, i.e., user key KEY-E == user key KEY-B, then abort with the message "QKD protocol insecure"

[0204] 490 Assessment determined that the error rate QBER < CQBERIQKD, so that in reality parties A (sender 210) and B (receiver 220) would rate the quality as acceptable and commence communication using the determined key.

[0205] 491 parameters for configuring the quantum computer 105 in the

[0206] Vary control unit 250 by one optimization step, note parameters as "A and B did not notice anything".

[0207] 495 Assessment determined that the error rate QBER > CQBERIQKD, so that in reality parties A (sender 210) and B (receiver 220) would rate the quality as unacceptable and would attempt a new key exchange.

[0208] 496 parameters for configuring the quantum computer 105 in the

[0209] Vary control unit 250 by one search step, save parameter as "A and B have noticed a fault".

[0210] 497 Sending the new parameters to the verification assembly 100, so that it sets the new parameters in the quantum computer 105.

[0211] 500 Illustration of how to perform a cascading error correction

[0212] 2025-11-14 BD-G0121-24 WO 510 Comparison of sender-side and receiver-side bit groups as

[0213] Parts of the raw key with cascading to locate the faulty bit in the

[0214] Case of parity inequality

[0215] 520 Grouping representation of the representation 510

[0216] 530 Information about the parity allocation to the bit groups accessible to the eavesdropping party E

[0217] 540 Error correction that the receiving party B due to the cascading

[0218] can perform parity comparisons

[0219] 600 Plotting of different error rates in the attempt to eavesdrop by - in this case present - party E, 731, eavesdropping in the quantum channel, between sender A, 710, and receiver B, 720

[0220] 610 Error rate of the raw key during the key exchange between Party A, 710, and Party B, 720, i.e., before error correction, caused by eavesdropping by Party E

[0221] 620 Relative proportion of the key correctly guessed by party E.

[0222] 630 Approximation curve over a large number of runs of the relative proportion of correctly guessed key versus the error rate of the raw key

[0223] 700 Order for the implementation of a quantum communication protocol

[0224] 710 Transmitter A

[0225] 715 Random generator of bits / qubits

[0226] 717 Randomly controlled modifier of the generated qubits (e.g., randomly chosen base)

[0227] 720 Receiver B

[0228] 725 Measuring device for measuring the modified received qubit

[0229] 1.1 Randomly controlled modifier of the received qubits

[0230] 730 quantum channel

[0231] 731 Potentially existing wiretapping party E

[0232] 732 Quantum channel possibly altered by the possibly existing eavesdropping party E - due to the eavesdropped qubits

[0233] 740 Public Channel

[0234] 800 methods for conducting a quantum key exchange (QKD, methods for establishing quantum communication)

[0235] 810 Starting step

[0236] 820 Sending the sequence of qubits via the quantum channel

[0237] 830 Comparison of the transmitter sequence with the receiver sequence via the public channel

[0238] 840 Error handling, for example error estimation

[0239] 850 Query to check if QKD protocol-related error limit cQBER has been exceeded

[0240] The cQBER threshold of 860 for the selected QKD protocol is below the limit: the key is considered OK.

[0241] The cQBER of 870 for the selected QKD protocol has been exceeded: action must be taken.

[0242] Procedure 880 is complete

[0243] 902 Computer System

[0244] 903 processor

[0245] 905 Storage unit (Memory)

[0246] 907 Bus

[0247] 909 Network adapter

[0248] 911 Storage System

[0249] 913 External Devices

[0250] 2025-11 -14 BD-G0121 -24 WO 919 l / O interface

[0251] 1000 quantum computer systems

[0252] 1004 Quantum Cloud Service Provider (QCSP)

[0253] 1002 Quantenprozessor (Quantum Processing Unit, QPU), Quantencomputer, Quantenrechner

[0254] 2025-11 -14 BD-G0121 -24 WO - Bes chreibung< / esc>

Claims

Federal Printing Office GmbH BD-G0121-24 WO Claims 1. Verification assembly (100) for verifying the security of a quantum protocol, QKD, comprising: a quantum channel input (110); a forwarding quantum channel (130); a quantum channel output (120); a port (140) for a listening line (140) intended to be connected to a public communication channel; and a communication port (150) for a communication line (150) intended to be connected to an external device in bidirectional communication.

2. Verification assembly according to the preceding claim, further comprising: an internal qubit generator (103) provided for generating a probe qubit within the verification device (100); a quantum computer (105) (Q) provided for entangling a QKD qubit received via the quantum channel input (110) with the probe qubit; an internal qubit sensor (107) provided for internally measuring the entangled resulting output probe qubit.

3. Verification assembly according to the preceding claim, wherein the internal qubit generator (103) is configured to generate random probe qubits.

4. Verification assembly according to any one of the preceding claims, further comprising: An internal correlation device (111) designed to estimate a QKD key from the measured output probe qubit and the captured signal at the listening line (140). 2025-11-14 BD-G0121-24 WO - Claims 5. Verification assembly according to any one of the preceding claims, further comprising: A communication line (150) intended to communicate the estimated QKD key externally and to receive request data from externally in order to control a variation of the parameterization of the quantum computer (105) via the communication line (150).

6. Arrangement (200) for verifying a quantum communication protocol, comprising: a verification assembly (100) according to any one of claims 1 to 5; a transmitter (210) (A) configured to execute the quantum communication protocol to be verified on the transmitter side; a receiver (220) (B) configured to execute the quantum communication protocol to be verified on the receiver side; and a control device (250) configured to communicate bidirectionally with the transmitter (210), with the verification assembly (100) and with the receiver (220).

7. Arrangement (200) for verifying a quantum communication protocol according to the preceding claim, wherein the control device (250) further comprises: Transmitter control line (261) between transmitter (A) (210), and control device (250), wherein the transmitter (A) (210) implements a quantum protocol to be verified; Input line (263) for monitoring the public communication channel (240), in communicative contact with the connection point (140) of the verification assembly (100), which connection point (140) is communicatively connected to the public communication channel (240); Transmit and receive line (265) between the verification assembly (100) and the control unit (250), which transmit and receive line (265) is in communicative contact with a connection point (150) of the verification assembly (100), so that bidirectional communication between the control unit (250) and the verification assembly (100) is enabled; Receiver control line (267) between the receiver (220) (B) and the control device (250), which receiver control line (267) enables a communicative connection between receiver (B) (220) and control device (250).

8. Procedure for verifying a QKD protocol to be tested, comprising: 2025-11-14 BD-G0121-24 WO - Claims Providing an arrangement (200) for verifying a quantum communication protocol according to one of the preceding claims 6 or 7; Execution of the QKD protocol to be tested between the sender (210) and the receiver (220) with the verification assembly (100) interposed communicatively in the quantum channel (230), wherein a sender-side qubit in a quantum computer (105) is entangled in the verification assembly (100) such that the sender-side qubit is forwarded to the receiver and the entangled qubit resulting within the verification assembly (100) with the sender-side entangled qubit is stored; Determining a receiver-side raw key (KEY-B) and its quality (QBER) as well as a verification assembly-internal raw key (KEY-E); Determining a receiver-side user key (KEY-B-USE) and a verification module-internal user key (KEY-E-USE); Discard the QKD protocol under test if the receiver-side user key (KEY-B-USE) and the verification module-internal user key (KEY-E-USE) match. 2025-11-14 BD-G0121-24 WO - Claims

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